1. Field
The disclosed embodiments relate to a method and an apparatus for work-piece levelling and for workpiece machining according to the preambles of the independent patent claims. Such a method has been known from U.S. Pat. No. 6,611,731.
2. Brief Description of Related Developments
In very precise numerically controlled machine tools, such as a laser ablation machine, manufacturing accuracies down to some few micrometers can be achieved. Usually, however, such a precise machining is only useful when the workpiece position is of at least the same or similar accuracy, because then the very precise machining in accordance with machining data relating to an ideal mounting is not compromised by a real mounting that may have a translatory or rotatory offset against the ideal mounting.
Often, however, it is not possible to mount workpieces with a precision such that only tolerable mounting inaccuracies in view of possible machining accuracies are given. Particularly, rotatory offsets have strong effects. A twist of a workpiece in the mounting for as few as 0.01° will, in a workpiece with parts 200 mm remote from the mounting, lead to a displacement of those remote parts for about 35 μm from the ideal position, which is inacceptable.
For this reason, a method is to mount the workpiece as precisely as possible, then to measure the workpiece in the mounting for detecting translatory and/or rotatory displacements/offsets of the workpiece in the real mounting in comparison with an ideal mounting. Then, in accordance with the detected offset, the machining data relating to the ideal mounting are numerically transformed towards the real position. Then, without changing the mounting, workpiece machining in accordance with the changed data is made. So, finally, one does not bring the real workpiece towards the ideal data. Rather, vice versa, the ideal data are transformed towards the real workpiece. In this manner, mounting inaccuracies can be eliminated to some extent.
FIG. 1 a shows the situation schematically in a two-dimensional cut in a machine coordinate system x-y. Assumed is a body 9 (for example a turbine blade) to be provided with a hole 8-1, 8-2 (for example for cooling fluid) that requires precise positioning. Item 1 is the surface of the body in the ideal mounting, 8-1 the (future) hole in the ideally mounted position. Item 2 is the surface in real mounting, and the (future) hole 8-2 is at a displaced position. The translatory displacement is shown, as an example, by arrow 3a at the lower left corner, wherein said arrow constitutes a displacement vector with x- and y-components. The rotatory displacement is indicated by angle 3b. 
In a three-dimensional space the relationships are qualitatively the same. The displacement vector is, however, a three-dimensional quantity, and one has three degrees of freedom for angular displacement and, thus, altogether six quantities to be determined.
FIG. 1b shows the interrelation of the ideal and the real coordinate system. System xi-yi-zi symbolizes the ideal coordinate system which may, for example, be the machine coordinate system. System xr-yr-zr symbolizes the real coordinate system. The latter is displaced against the former by displacement vector vv=(xv, yv, zv) by translation and is rotatory displaced around plural axes by angles λv, βv, φv. The relation between a point/vector vi in the ideal coordinate system and a point/vector vr in the real coordinate system can mathematically be expressed as follows:vr=k*D(λv,βv,φv)*vi+vv, 
wherein D(λv, βv, φv) in three-dimensional computings is a 3*3 matrix effecting the rotation, and k is a scaling factor. By said formula a coordinate point in the ideal system can be transformed into a point in the real system, when xv, yv, zv, λv, βv and φv are known. Advantageously, the quantities xv, yv, zv, λv, βv and φv are defined in respect to the ideal coordinate system (machine coordinate system), because this is initially known. In this context it is pointed out that a plurality of descriptions of the transformation between real and ideal coordinate system exist, particularly along other translatory and rotatory axes. These may be selected such that the transformation requires less than the mentioned six parameters (3 translatory (xv, yv, zv), 3 rotatory (λv, βv, φv). However, then, these axes must be determined and described so that computational workload does not decrease, but only changes. When, however, in the workpiece or in the work to be made certain symmetries are given (such as in a circular bore hole), a lower number of parameters for describing the transformation may be sufficient.
The determination of the translatory displacement 3a and of the rotatory displacement 3b can be made such that the real position of plural reference points 2-1, 2-2, 2-3 of the body in its real mounting is measured. The measured values are compared with the expected ideal values of the reference points 1-1, 1-2, 1-3 according to the ideal mounting. In this way a real position and attitude of the body can be determined. With reference to the known ideal position and attitude, then, translatory and rotatory displacement can be determined which may be applied to the machinig data. The US patent cited above describes such a method.
The described method has two disadvantages:
On the one hand, one will hardly ever hit the reference points precisely when measuring them, because, usually, they are not marked, so that a measuring probe can approach them only in their ideal coordinates (because the real ones are unknown), so that a found error may, at least in parts, not be caused by a displaced mounting, but rather by measuring a not correctly hit reference point. To some extent, this problem can be overcome by suitably selecting reference points and access direction of the points and by appropriately designing the algorithm (such as iteration when large deviations are found). However, this makes the method intricate and it must be individually adjusted for every workpiece.
On the other hand, it may happen that a reference point, in reality, does no longer exist. This occurs particularly when workpieces are repaired or in maintenance (such as turbine blades) after many hours of service time. They may show wear or crude mechanical impact (such as bird strike), so that, caused by smaller or larger surface defects, the reference point has disappeared. Surface defects may also be material aggregations because of deformed material or some kind of agglomeration. FIG. 1a shows at 2a a dent in the surface and, accordingly, reference point 2-3 is “in the air” and cannot be measured. When, instead of this reference point, nevertheless something is measured (e.g. the bottom of dent 2a), something wrong is measured and accordingly wrong are the values derived therefrom.
Further relevant prior art are DE 19631620 and DE 3119566.